27" iMac as External Display in Mavericks

I'm using a late 2009 27" iMac and a mid-2010 MacBook Pro. I occasionally use a mini displayport cable to connect them and the iMac works as an external display for the MacBook Pro. From past experience, I knew the brightness control didn't work if the iMac is running Snow Leopard, but it worked OK with Mountain Lion.

I recently upgraded both to Mavericks. I'm able to get the iMac to show up as a display, but if I touch the brightness key on the iMac keyboard to adjust the brightness, the iMac display goes black and the whole machine is stuck. Even after I disconnect the cable, the iMac doesn't respond to anything except a forced shutdown (hold down power key in the back of the machine). After a reboot, the top half of the iMac display backlight flickers for a minute or so. Fortunately the flickering is temporary.

I have Snow Leopard on a second partition, so I booted the iMac into that. The iMac works as an external display, but the brightness keys do not functions (just as before). I had a Mountain Lion recovery partition left over on the iMac as well, so I installed Mountain Lion on yet another partition (clean install) and this appears to fix the issue for me. I'm writing this on the iMac screen and the brightness controls work as expected. Hopefully Apple will make note of this issue in Mavericks and a future patch will fix the problem.

I'm posting this to document the issue so that Apple may fix it and so that others who encounter the same problem may also be able to work around it.

Being able to use the iMac as an external display is a pretty big deal to me. One possible upgrade path for me from the iMac is to buy a new MacPro and use the iMac as a display on that. For that to work, the iMac has to be able to function as a display and the display has to be in good condition (that flickering after the reboot was really worrying as it made me suspect the screen was damaged somehow).